The present invention relates to an integrated optical proximity coupler intended to separate or combine two signals of different wavelengths. More particularly, the invention relates to such a coupler, exhibiting predetermined pass bands surrounding these wavelengths in the sense which will be given below in this discussion.
We are familiar with proximity couplers such as the one which is schematically represented in FIG. 1 of the attached design in the present patent application. Such a coupler is currently being utilized in the multiplexing/demultiplexing of two signals of different wavelengths.
The coupler of FIG. 1 comprises two waveguides 1,2 integrated by an ion exchange such as the thallium ion, for example, in a glass substrate 3 or in a crystalline substrate. The exchange of ions is carried out across a mask which defines the form of the waveguides. These consist of straight inter-action segments 1.sub.1, 2.sub.1, parallel and adjoining, and approach parts 1.sub.2, 1.sub.3, and 2.sub.2, 2.sub.3 respectively, connected to entries/exits or "ports" 4, 5, 6, ports 5, 6 being separated by a distance which is a function of the diameter of the coated optical fibers (not represented) which are connected to these ports.
Thus, signals of wavelengths .lambda..sub.1, and .lambda..sub.2 (.lambda..sub.1 &lt;.lambda..sub.2) entering by port 4 exit separately through port 6 and 5, the signal at one of the wavelengths passing into waveguide 2 because of the passage of evanescent waves in the substrate between the straight inter-action segments 1.sub.1 and 2.sub.1 of the guides, in particular. The response curve of such a coupler typically takes the form represented in FIG. 2, which makes visible two attenuation peaks centered on the wavelengths .lambda..sub.1 =1310 nm and .lambda..sub.2 =1550 nm, as is the case for example, when one utilizes such a coupler for optical fiber telecommunications. It is clear that the signal at wavelength .lambda..sub.2 =1550 nm is highly attenuated at port 6, while the signal at wavelength .lambda..sub.1 =1310 nm is practically not, and inversely at port 5. One can thus separate, for example, a video signal carrier signal from an audio-signal carrier signal transmitted in the same optical fiber. The same signals can be reunited in the same fiber by advancing in the opposite direction in the coupler of FIG. 1.
The transfer of signal from one guide to the other is governed by the relationship: EQU P=sin.sup.2 (C(s,.lambda.),L)
where P is the power transferred from one guide to the other, C(s,.lambda.) is a coefficient of coupling which depends upon the separation (or distance) s between the axes of the straight interaction segments of the guides, and upon the wavelength of the signal utilized, and L the length of the straight interaction segments.
In order to obtain a good separation of the signals it is appropriate that: ##EQU2## n and m being integers, one odd, the other even. We call such a coupler, a coupler of the type (n,m). By choosing the parameters which occur in these equations in such a way that they can be satisfied, we find the entire power of the signals of the wavelengths and .lambda..sub.1 and .lambda..sub.2 at exits 6 and 5 respectively, without having a fraction of one interfere with the other at each of the exits.
This is what we observe practically speaking in the graph of FIG. 2 where it appears that for wavelengths .lambda..sub.1 and .lambda..sub.2, the fraction of one signal which strays to the other is reduced by 30 dB approximately. Around these two wavelengths, the attenuations are less strong and one is hence led to define a "pass band" (bandwidth) around each of the two wavelengths in order to characterize the selectivity of a proximity coupler. In this respect there exists a need for a means which would permit the adjustment of this pass band. Such a need exists, for example, when one wishes to have at one's disposal a pass band of increased width around one of the two wavelengths, permitting one to tolerate the utilization of a source of signal whose wavelength can be defined less precisely around the utilization wavelength. Such a tolerance can be advantageous from the point of view of the cost of the source in question.
The present invention has as its object, therefore, furnishing a proximity coupler of the type described above, conceived in such a manner as to exhibit a predetermined pass band for one of the two signals which are to be separated to combined in the coupler.
We attain this object of the invention, as well as others which will appear upon reading the description which will follow, with an integrated optical proximity coupler for the separation or combining of two signals of wavelength .lambda..sub.1 and .lambda..sub.2 in increasing order, comprising two waveguides imbedded in a substrate and exhibiting a coupling region of length L establishing coupling coefficients C.sub.1 and C.sub.2 for signals of wavelengths .lambda..sub.1 and .lambda..sub.2, respectively, characterized in that the length L of the coefficients C.sub.1 and C.sub.2 are linked by the relationships: ##EQU3## where n and m are uneven and even integers respectively, chosen in such a manner as to establish a pass band of predetermined width around at least one of the wavelengths .lambda..sub.1 and .lambda..sub.2.
We can thus adjust a pass band to a given tolerance affecting the wavelength of one source and one of the two signals, the deference to this tolerance being potentially advantageous for reasons of cost of manufacture of said source, for example.
In accordance with a preferred embodiment of the invention, m=n+1, the waveguides of the coupler being singlemode, symmetrical and integrated by ion exchange in a substrate. In a particular application to optical fiber telecommunications, .lambda..sub.1 =1310 nm and .lambda..sub.2 =1550 nm, the width of the pass band around the wavelength .lambda..sub.2 being greater than the width of the band around the wavelength .lambda..sub.1. The pass band of predetermined width is obtained by adjusting the length L and the separation of the two guides in the coupling region.